Hydrogen embrittlement behaviors of a high-Mn TWIP (twinning induced plasticity) steel with various grain sizes from coarse grains to ultra-fine grains were studied by hydrogen pre-charging and subsequent slow stain rate tensile tests. The results of the tensile tests showed that the yield strength and tensile strength were not influenced by hydrogen-charging, whereas the total elongation reduced with hydrogen-charging in coarse-grained specimen but no change in the ultrafine-grained specimen. Fracture surfaces showed dimple patterns in all specimens. The present results suggested that the grain refinement suppressed hydrogen embrittlement in the high-Mn TWIP steel, even though the diffusible hydrogen content increased by grain refinement.